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hello all,

i have declared a global descriptor as below:


gdt:

null_descriptor:

dw 0 ; seg_length1_15
dw 0 ; base_addr0_15
db 0 ; base_addr16_23
db 0 ; dflags
db 0 ; access
db 0 ; base_addr24_31

code_descriptor:

dw 0xFFFF ; seg_length1_15
dw 0 ; base_addr0_15
db 0 ; base_addr16_23
db 0x9A ; dflags
db 0xCF ; access
db 0 ; base_addr24_31

data_descriptor:

dw 0xFFFF
dw 0
db 0
db 0x92
db 0xCF
db 0

video_descriptor:

dw 0x00EF
dw 0xb800
db 0
db 0x92
db 0xCF
db 0

gdt_end:

gdt_descriptor:
dw gdt_end - gdt - 1
dd gdt



But, when i try to move the character 'a' into the video segment, it fails. The coding for moving is also given.


mov ax, 10h
mov ds, ax
mov byte [ds:0], 'A'
mov byte [ds:1], 1BH


Please, help me out of this problem.


Thanks in advance.

Is your code located at segment 0x0000? Rememeber, GDTR.base and GDT.base are absolute. You've got to add CS<<4 to them to work.

it is the gdt register, not the gdt descriptor.

Cheers,
Adrian

also, from the code you give, your video segment if 0x18 not 0x10

also, from the code you give, your video segment if 0x18 not 0x100x10

also, from the code you give, your video segment if 0x18 not 0x100x10

You need properly switch to protected mode, I recommend you get some step-by-step tutorial. There are some well protected mode tutorials at:

http://osdev.neopages.net/tutorials.php?cat=4&sort=1

The author Chris Giese write the best protected mode examples I have found, but I don´t remeber where I get these examples. Ask him!

There is also an example at:

Intel Architecture Software Developer’s Manual Volume 3: System Programming
CHAPTER 8 PROCESSOR MANAGEMENT AND INITIALIZATION
8.9. INITIALIZATION AND MODE SWITCHING EXAMPLE

You can request a copy at the INTEL web site!

pepito

OK, you need at least this steps to switch to protected mode:

1) Create a GDT (Your code have one!)

2) Load the GDTR with the base and limit of your GDT:

   lgdt [gdt_descriptor]      

   (but be careful about what Adrian say!)

3) Set the PE bit at CR0 register:

   mov eax, cr0            
   or al, 1
   mov cr0, eax

4) Do a far jump to enable protected mode, you need a label before your PM code:

   jmp 8:some_label   ; Code descriptor

   [BITS 32]
   some_label:

   ; Update data segment registers

   mov ax, 16 ; Data descriptor
   mov ds, ax
   mov es, ax
   mov fs, ax
   mov gs, ax

   ; HERE YOU CAN TRY YOUR VIDEO CODE !!!

   mov ax, 24 ; Video descriptor
   mov ds, ax
   mov byte [ds:0], 'A'
   mov byte [ds:1], 1BH

pepito